The function of tails has somewhat stumped scientists over the years, but Pacheco and colleagues have now determined that female rats' tails play a very important role in the rodent mating process.

Female rats use their tails to direct, stimulate and balance their male partners, the scientists found.

Tails have been known to provide animals with additional sensory perception, since a tail can "feel" around itself, and help with heat regulation, balance and navigation.

Cats, dogs and rabbits

According to Pacheco and team the new findings, which add mating facilitation to the tail's known list of functions, could also apply to cattle, cats, dogs, rabbits and certain primates, such as the spider monkey.

Every part of the tail appears to be important.

Pacheco says the base of the female's tail facilitates, modulates and even permits the male's lateral mounting, which guarantees that the penile tip will find the vaginal opening, while the tip of her tail then offers the male stimulation.

His team surgically removed both female and male rat tails and then observed how the removals affected their mating.

They then join the female and initiate rhythmic pelvic thrusts, which lead to the actual mating.

Neither the males nor the females found the tail-less subjects to be any less attractive.

Males without tails seemed to move a bit awkwardly, but it was the tail-less females that really threw off the normal pattern, according to the study.

The couples went through similar processes, but when female partners lacked tails, the males performed fewer mounts, seemed to have trouble finding their way and completing the mating process.

Pacheco points out that the males cannot see what they are doing, so the female's tail helps to guide their movements.

"We think that the information supplied by the tail's sensory machinery modulates the pelvic thrust movements, helping the mechanism for the ejaculatory process as well as for the correct deposit of the semen," he says.

The researchers do not believe tail length matters, since males tend to have longer tails than females and tail length varies among different types of rats.

Human implications

Dr Anders Agmo, chair of the Department of Psychology at the University of Tromsø, Norway, heard of Pacheco's research after he gave a talk on neurobiology in Mexico.

He says that his first reaction was one of "surprise and admiration".

"Surprise, because it had never occurred to me to ask any questions about the importance of the tail for rat sex behaviour and admiration because I immediately realised that it was an interesting and perhaps even important question," says Agmo.

He adds that the findings could have human applications. Although people obviously don't have tails, much of the rat-tail information is transmitted through the spinal cord, and rat and human spinal cords are quite similar in their basic structure.

"Several human sexual dysfunctions are related to sensory feedback from the genitals and the responses produced by this feedback," he says, explaining that premature ejaculation is one of the most common problems related to reduced reactivity to sensory feedback.

"If we could achieve a complete understanding of these neural mechanisms in a rat, we would certainly be able to better understand and treat human disorders," says Agmo.